Compact, robust and inexpensive laser sources of visible light have many applications in consumer products, industrial processes and scientific instruments. Electrically pumped, edge-emitting semiconductor lasers, usually referred to by practitioners of the art as diode-lasers, are compact, robust and efficient. However, the most common commercially-available diode-lasers are lasers that emit light in the red region of the visible electromagnetic spectrum or the near infrared (NIR) region of the invisible electromagnetic spectrum. A few commercial suppliers are able to provide diode-lasers that emit fundamental radiation in the blue region of the visible electromagnetic spectrum. There are no commercially available diode-lasers that emit fundamental radiation at any wavelength between the red and blue regions of the visible electromagnetic spectrum.
One means of providing laser radiation in the spectral regions between red and blue is to frequency-double radiation from a diode-laser having a wavelength in the NIR region of the spectrum. In some prior-art arrangements for doing this, the output of a diode-laser is collected by suitable optics and directed into a frequency converter (frequency multiplier). This may be described as a direct frequency-doubling approach. As the peak power-output of a diode-laser output is relatively low (at most, a few Watts), a highly efficient optically nonlinear crystal is needed for frequency-doubling. Examples of such crystals are periodically poled (PP) crystals of lithium tantalate (LT), lithium niobate (LN), and KTP. Typically, a PP crystal forms a narrow waveguide, so that diode-laser radiation launched into the crystal remains at a high intensity level for an extended length, for example, several centimeters. Such an extended path with a high intensity is not achievable with a focused free-space beam.
A problem with this approach is that complex free-space optics are needed to launch the diode-laser output into the PP crystal. This generates additional problems due to a requirement for precise alignment and a high degree of mechanical stability. This also drives the cost of such a frequency-doubled diode-laser beyond a level tolerable in most applications. To put this in perspective, low-cost commercially available, diode-laser driven, green-light sources for use in laser pointers and laser displays are actually intra-cavity frequency-doubled solid-state lasers. In these lasers the NIR diode-laser optically pumps a crystal of a solid-state gain-medium to generate NIR fundamental radiation. This fundamental radiation is frequency-doubled by a relatively small (a few millimeters long), relatively inexpensive optically nonlinear crystal located in the resonator of the solid-state laser. This may be described as an indirect frequency-doubling approach. While relatively inexpensive, these lasers are still somewhat vulnerable to misalignment by mechanical shock or temperature cycling. Accordingly, cost issues aside, in order to address stability and alignment problems in providing visible light from the fundamental-radiation of an NIR diode-laser, there is a need for a monolithic device that provides direct frequency-doubling in a PP optically nonlinear crystal without the need for free-space coupling optics.